Scripsit Andy Dingley:
> Using px or pt also had the disadvantage that an IE user cannot
adjust the font size to their preference.
Px and pt units have a defined meaning, which it's wrong for FF to
make flexible like this. Understandable, as it was a reaction to
clueless web coders setting fonts this way rather than in ems, but
still wrong.
It's wrong indeed, but px and pt don't really have that well-defined
meanings, and - more importantly - browsers generally deviate from the
defined meaning of pt.
The px is defined as a pixel, but what _is_ a pixel? It's a basic unit of
rendering on screen, though not atomic unit (since we have subpixel
rendering). It's physical size naturally varies.
But please read what the CSS 2.1 draft babbles about "reference pixel" at
http://www.w3.org/TR/CSS21/syndata.html#x39
They say: "If the pixel density of the output device is very different from
that of a typical computer display, the user agent should rescale pixel
values." What is "very different", and rescale how? Probably this is meant
to relate to "pixels" (dots) in printing, not on screen, but it's not clear
at all.
I gets worse with pt. Nominally, 1pt = 1/72 of an inch, by definition. This
means that 1pt is based on the inch, which in turn is based on the meter
(1in = 25.4mm = 0.0254 m, by definition), which in turn in based on
universal physical constants.
Yet, what happens if set something in pt (or in or mm)? In most contexts,
browsers convert the quantity to pixels, usually by the formula 1in = 96px.
This means that pt, in, or mm coincide with their defined physical meanings
only by accident, when the display device happens to use a pixel size of
1/96px. This might well be your monitor's default resolution, so if you make
something 1in or 96px wide and measure it with a rules, it's correct. But if
you then change your monitor properties to use a different resolution - and
most modern monitors have a few resolutions to choose from - you get
different results. You will see that a point is no longer a point, an inch
is not an inch, and a millimeter is not a millimeter.
People who do something remarkably stupid like setting the overall font size
to 8pt usually have no clue of this. They think the text is quite legible,
without realizing even the _physical_ variation of the size, not the mention
different viewing conditions and people's eyesight.
--
Jukka K. Korpela ("Yucca")
http://www.cs.tut.fi/~jkorpela/ 
